Biodiesel production capacity worldwide is increasing every year with regulatory and socioeconomic motivations for renewable energy. One mole of glycerol is generated along with each mole of triglyceride converted to biodiesel. As the result of biodiesel growth, large amount of glycerol are produced and are available to the marketplace. Various sources [1, 2] have reported that the price of glycerol has been lowered by its large availability, and even credits are given for selling the crude glycerol. Development of value-added chemicals from glycerol, the co-product with biodiesel, is necessary to help sustain the biodiesel industry. Glycerol is listed among the 12 top building block chemicals from renewable biomass by the United States Department of Energy (DOE) [3].
Lactic acid (LA) has been viewed as a “commodity chemical sleeping giant”, owing to its multiple reactive functionalities that make it readily convertible to other important commodity chemicals, such as ethyl lactate and poly(lactic acid). Currently, LA is primarily produced via fermentation of carbohydrates, which suffers from low productivity and efficiency and might not be sufficient to meet the increasing demand of LA in the chemical industry [4].
To date, almost all reported LA studies using chemical methods instead of biological methods of fermentation were conducted using homogeneous alkaline solutions; more specifically, good LA yields were achieved with homogeneous solution of either NaOH or KOH. High corrosiveness caused by NaOH and KOH under hydrothermal conditions constrained the allowable concentration of alkaline, and consequently concentration of glycerol in the reactants. This limitation negatively influences the productivity of LA. Furthermore, there is no inexpensive separation process to remove NaOH/KOH from the products, which hinders the promotion of industrialization of the technology. Moreover, most processes developed for glycerol-to-PG conversion require an external supply of hydrogen, which is associated with additional purchase and handling costs. Furthermore, depending on the hydrogen source, the developed PG process, which should ideally be based on renewable resources, could become dependent on fossil fuels. These problems have hindered and continue to hinder the commercialization of value-added chemicals from glycerol.
Thus, new methods, systems and devices for the conversion of glycerol to value-added components such as lactic acid and propylene glycol are needed.